Cream butter and 25 g sugar till pale and fluffy, scraping sides and bottom of mixing bowl as necessary. Add egg yolks 1 at a time, plus vanilla extract with the last yolk. Beat thoroughly after each addition till whisk leaves distinct trail. Scrape down thoroughly.

Re-sift 1/4 of sifted flour into butter mixture. Fold with spatula till just even. Re-sift another 1/4 into mixture. Again, fold till just even, scraping down as necessary to ensure even mixing. Add milk in 2 batches, folding till just fully absorbed after each addition. Re-sift and fold remaining half of flour into mixture in 2 batches as before.